Project Summary: Existing treatments for major depressive disorder (MDD) may require several weeks to months to exert their maximal benefit. Ketamine, a high-affinity NMDA glutamate receptor antagonist, is an anesthetic and analgesic medication commonly used in pediatric and adult patients. Ketamine has antidepressant properties in animal models, and may have rapid antidepressant activity for patients with severe mood disorders. A recent placebo-controlled investigation in patients with treatment-resistant unipolar depression (TRD) showed robust antidepressant efficacy of a single subanesthetic dose of intravenous (IV) ketamine (0.5 mg/kg), findings which replicated a previous small pilot trial. Strikingly, a high proportion of patients maintained the acute response to IV ketamine for several days or longer. In this 4-year single-site study, we will conduct a triple-masked, randomized, parallel-arm active control investigation of the acute efficacy and safety of IV ketamine in TRD. Specific Aims: (1) To test whether a single IV infusion of ketamine has superior antidepressant efficacy compared to an active control agent (IV midazolam) in patients with TRD. (2) To characterize the durability of benefit and test whether IV ketamine is associated with superior antidepressant effects at the 7-day timepoint. (3) To examine the safety and tolerability of the interventions. The intent-to-treat sample is comprised of 64 TRD patients, conservatively defined as insufficient response to 7 3 adequate antidepressant trials in the current episode. After a medication-free period 7 2 weeks, patients are randomized to receive an infusion of either IV ketamine (0.5 mg/kg over 40 min) or IV midazolam (0.045 mg/kg over 40 min). Efficacy and safety is evaluated over the following 7 days. Nonresponders at the 7-day endpoint exit the study, while patients meeting response criteria are followed bi-weekly until relapse or for an additional 4 weeks. The primary efficacy measure is reduction in depression severity as assessed by the Montgomery- Asberg Depression Rating Scale (MADRS) scale 24 hours following IV infusion; additional outcome measures include response and remission rates, durability of benefit (7-day endpoint), side effects, and adverse events.